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# design of Brakes system

26 Mar 2023
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### design of Brakes system

1. UNIT 2 BRAKES A Brake is defined as a mechanical device, which is used to absorb the energy possessed by a moving device, system or mechanism by means of friction. The primary purpose of the brake is to slow down or completely stop the motion of a moving system, such a rotating drum, machine or a vehicle.
2. Brakes The energy absorbed by brake is converted into heat energy and dissipated to surrounding i.e. Heat Dissipation is a serious problem in brake applications. Brakes are classified into following 3 groups 1. Mechanical brakes – operated by means of levers, springs and pedals 2. Hydraulic and Pneumatic brakes – operated by fluid pressure such as oil pressure or air pressure 3. Electrical Brakes – Operated by eletro-magnetic forces.` Types of Mechanical brakes: Shoe brakes, Band brakes, Internal and External expanding brakes
3. Energy Equations The first step in design of a mechanical brake is to determine the braking torque capacity . The braking torque capacity depends upon the amount of energy to be absorbed by the brake.
4. Chapter 12
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8. Chapter 12
9. Numerical Problems
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11. Chapter 12
12. Block Brake with short shoe A block brake consists of a simple block, which is pressed against the rotating drum by means of a lever as shown in figure. The friction between the block and brake drum causes the retardation of drum. Application: On rail wheels. Angle of contact between block and brake is usually small < 45O. Since angle of contact is small, it results in uniform pressure distribution on block.
13. Free Body Diagram (Clockwise Rotation) Mt – Braking torque (N-mm) R – radius of brake drum (mm) μ - coff. of friction N – Normal reaction (N) The dimensions of block are determined by N= plw Here p – permissible pressure between block and brake drum (N/mm2) l= length of block w = width of block A Narrow Block (lesser w) drawback-> Size – large A Broad Block (Higher w) drawback -> Pressure distribution is non-uniform
14. Free Body Diagram (Clockwise Rotation) Assumptions The width of block should be optimum between 2 limits given by: (1/4 )* drum diameter < w < (1/2)* drum diameter
15. Free Body Diagram (Clockwise Rotation) Considering the equilibrium of forces in vertical and horizontal directions: Taking moments of forces acting on lever about the hinge point O, Depending upon magnitude of coeff. Of friction (μ), location of hinge pin (c), there are 3 different cases: 1. a > μ c (Desirable condition) Partially self energising –P required for brake 2. a = μ c (Non-Desirable condition) The break is self-locking. P=0 3. a < μ c (Non-Desirable condition) P= Negative This is a dangerous operating condition, resulting in uncontrolled braking and grabbing. The brake is non-controllable by operator as he can not apply it.
16. Free Body Diagram (Anti-clockwise Rotation) P (Braking effort) also depends upon the direction of rotation of brake drum: In design, the objective will be to design for: • Smaller braking effort • Avoid dangerous and undesirable braking conditions The main disadvantage of block brake is the tendency of brake drum to bend under the action of normal force. The remedy is to use two symmetrical blocks at opposite sides of brake drum.
17. Fig. 12.5
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22. Fig. Free Body Diagram of Forces
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24. Block Brake with Long Shoe • The angle of contact in this case is > 450. • For the short shoe brake normal Reaction (R) and frictional forces are assumed to be concentrated at the center of shoe. • This assumption is not applicable for brake with long shoe brake.
25. Block brake with long shoe Equation similar to Block Brake with short shoe
26. Fig. 12.16 Internal Expanding Brake Internal expanding brake Application: Vehicle, conveyor, belt
27. Fig. 12.17 Free Body Diagram of Forces Internal expanding Force diagrams
28. Chapter 12
29. Chapter 12 Torque
30. Chapter 12 Anticlockwise direction
31. Chapter 12 FORWARD AND REVERSE MOVEMENT
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38. Fig. 12.19
39. Band Brake A simple band brake consists of a flexible steel strip lined with friction matrial, which is pressed against the braking drum. When one end of band passes through the fulcrum of the actuating lever, the brake is called simple band brake. The working of steel band is similar to that of a flat belt operating at the zero velocity
40. Free body diagram Ratio of band tensions are given by: P1= Tension in tight side of the band (N) P2= Tension in slack side of the band (N) μ = coeff of friction between friction lining and brake drum Θ = Anlgle of wrap (rad) Mt = torque capacity of brake (Nmm) R = radius of brake drum (mm) Considering the forces acting on lever and taking moments about the pivot:
41. Differential Band Brake (a) Construction (b) Free Body Diagram A band brake is called differential band brake when neither end of band passes through the fulcrum of actuating lever. Such brakes are designed for the condition of self locking
42. Chapter 12
43. Advantages of self-locking: Although self-locking is undesirable in speed control brakes. It is used to advantage in Back-stop mechanism. A back-stop brake is device, which is used to prevent the reverse motion of drum would have harmful effects: Applications: • Bucket conveyors • Hoisting application • Material handling
44. Fig. 12.24
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47. Chapter 12 Disk Brake A disk brake is similar to a plate clutch except that one shaft is replaced by a fixed member. Front wheel of motor cycle. Vented disc brakes have a set of vanes, between the two sides of the disc, that pumps air through the disc to provide cooling Brake Pad
48. Chapter 12 Caliper Disk Brake The disc brake is same like the brakes on a bicycle. In a disc brake, the caliper which squeeze the brake pads the rotor instead of the wheel, and the force is transmitted hydraulically instead of through a cable. Friction between the pads and the disc slows the disc down. Ex: lift trucks, farm machinery, front wheel of motor cycle Single plate clutch equation is used for disk brakes.
49. Disk Brake Annular pad The dimensions of annular pad are as follows: Ro = Outer radius of pad (mm), Ri = Inner radius of pad (mm), θ= angular dimension of pad (radians) Since the area of pad is comparatively small, it is assumed that pressure on the friction lining is uniform, The braking torque capacity according to uniform pressure theory is given by: Two shapes of pads 1. Annular 2. circular
50. Chapter 12 Disk Brake with Annular Pad
51. Disk Brake with Circular Pad Friction radius of circular pad is given by: Rf = δe e = distance of pad centre from axis of disk (mm) And values of δ are taken from table R/e δ 0.0 1.0000 0.1 0.9833 0.2 0.9693 0.3 0.9572 0.4 0.9467 0.5 0.9375 R- radius of circular pad (mm)
52. Chapter 12 Thermal considerations •The energy absorbed by the brake is converted into heat, which increases the temperature of rubbing surfaces. •As the temperature increases, the coefficient of friction decreases that adversely affects the torque capacity of brake. •At high temperature there is rapid wear of friction lining which reduces the life of the lining •The temperature rise should be kept within permissible limits. •If it is assumed that all the heat generated during braking operation is absorbed by the brake drum assembly, then: Δt = E/(mc)
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